In many digital data transfer systems there is a need to synchronize clock devices of different network elements with respect to each other in such a way that phase values, and possibly also time values, maintained in different network elements are sufficiently close to each other. In other words, each of the clock devices of different network elements should indicate a phase value, and possibly also a time value, that is common to all network elements in question. A common time value is usually called as “wall clock time” or “universal time”. In this document the term “synchronization” means either phase synchronization or time synchronization or both. The above-mentioned network elements can be, for example, user terminals that are connected to a digital data transfer network, routers of a digital data transfer network, or base stations of a cellular mobile network. A digital data transfer network itself may need synchronization between network elements. For example, in a new generation cellular mobile network, a precondition for successful data transmission between base stations and a mobile phone that moves from a coverage area of a certain base station to a coverage area of another base station is the fact that the base stations follow a common phase value, and possibly also a time value, with a sufficient accuracy. It is also possible that a user application that is based on a digital data transfer system may require synchronization. For example, in an application for monitoring a power grid, measurements having accurate timing information are transferred to a central control station where informed decisions can be made in possible emergency situations.
In a solution according to the prior art clock devices of network elements that are to be synchronized with respect to each other are synchronized with the aid of a timing signal that is received from a GPS-satellite (Global Positioning System). In the future, it may be possible to use also the European Galileo-system and/or the Russian GLONASS-system together with or instead of the GPS-system. A GPS-receiver and an antenna system increase, however, the component and manufacturing costs of a network element. Furthermore, the antenna system connected to the network element has to be situated in such a way that the GPS-signal can be received with a sufficient power level. A network element such as a router or a switching centre can be located in underground premises. In this case, there is a need to build a cabling system with the aid of which a received GSP-signal can be delivered from a terrestrial antenna to the network element located in the underground premises.
In another solution according to the prior art, network elements are arranged to transmit timing messages, e.g. timestamps, to each other within dedicated protocol data units that are used for timing purposes in order to achieve mutual synchronization. An example of a synchronization method according to the prior art is presented in IEEE 1588 specification (Institute of Electrical and Electronics Engineers). The synchronization method requires multiple exchanges of different timing messages. A protocol data unit (PDU) can be, for example, a data transfer packet, a data transfer frame, or a data transfer cell. A data transfer packet can be e.g. an IP-datagram (Internet Protocol), a data transfer frame can be e.g. a Frame Relay-frame, and a data transfer cell can be e.g. an ATM-cell (Asynchronous Transfer Mode). Transmission of protocol data units that are dedicated for timing purposes consumes, however, data transfer capacity that would/might be needed for payload data.